(1) Field of the Invention
The present invention relates to a subchamber type combustion chamber for a diesel engine. More specifically, the present invention relates to a subchamber type combustion chamber for a diesel engine, which can reduce fuel consumption rate and combustion noise.
(2) Description of Related Art
Conventionally, in a subchamber type combustion chamber for a diesel engine, a subchamber is communicated with a main combustion chamber through a nozzle hole, a fuel injection nozzle faces into the subchamber, and a glow plug extends into the subchamber (for instance, see Japanese Patent Application Laid-Open (JP-A) No. 2001-248444 (see FIG. 1)).
According to such a subchamber type combustion chamber, a fuel is injected from the fuel injection nozzle to compressed air pushed from the main combustion chamber through the nozzle hole in a compression stroke to cause premixed combustion in the subchamber. By the combustion pressure, an uncombusted gas is injected from the subchamber through the nozzle hole into the main combustion chamber and is then mixed with the compressed air in the main combustion chamber to be combusted in the main combustion chamber. The increase speed of the combustion pressure in the main combustion chamber can be lowered to reduce combustion noise.
In such a subchamber type combustion chamber, a heat generating portion of the glow plug includes a rod-shaped portion and a projecting portion projecting from the rod-shaped portion. The projecting portion typically has a semispherical surface. The percentage of value (P/D) obtained by dividing projection dimension (P) of the projecting portion from an end of the rod-shaped portion by diameter dimension (D) at the end of the rod-shaped portion is approximately 50%.
<<Problems>>Fuel consumption rate and combustion noise are high.
When the percentage of value (P/D) obtained by dividing projection dimension (P) of the projecting portion from the end of the rod-shaped portion by diameter dimension (D) at the end of the rod-shaped portion is approximately 50%, fuel consumption rate and combustion noise are high.
This reason is assumed as follows.
That is, the surface area of the projecting portion becomes too large, so that a large amount of combustion heat escapes from the subchamber through the projecting portion during engine operation, resulting in great heat loss. Consequently, fuel consumption rate is high. In addition, abrupt premixed combustion due to delayed ignition occurs, resulting in high combustion noise.